Growth process modification to improve the quality of Al0.45Ga0.55N
Hydride vapor phase epitaxy (HVPE) offers the possibility to efficiently grow thick layers of AlN, GaN and AlxGa1-xN at high growth rates. Binary AlN and GaN layers with a thickness above 50 µm are already commercially available. They serve as the basis for growth of devices like UV-LEDs. In particular, AlxGa1-xN in a medium composition range (x≈0.5) would be an ideal base layer for the fabrication of UV-B LEDs emitting at wavelengths between 280 – 320 nm. However, such HVPE grown ternary layers are not yet available mainly due to the bad layer morphology that suffers from polycrystalline growth and composition inhomogeneity.
The improvement of HVPE grown Al0.45Ga0.55N crystal quality is a current research topic at FBH. For that purpose, the sapphire substrate is patterned with parallel trenches which then are laterally overgrown (epitaxial lateral overgrowth, ELOG) by material growing from the top of the ridges. This method is expected to improve the material quality and especially to avoid crack formation. As shown in Fig. 1a, distinct polycrystalline growth occurs from the sidewalls of the trenches. Facets are offered on the stepped surface on which the deposition of Ga-rich material is favored resulting in inhomogeneity in the Al-distribution in the Al0.45Ga0.55N layer. To achieve the aim of a coalesced flat layer by growth from the top of the ridges ways to suppress growth in the trenches were investigated. The deposition of a 500 nm thick AlN layer prior to growth was found to support the Al0.45Ga0.55N deposition on the top of the ridges, but the material grown from the sidewalls of the trenches still dominates the surface morphology (Fig. 1b). The surface mobility of the precursors on the sapphire substrate can be influenced by the total pressure during growth. A decrease in total pressure from 800 hPa (Fig. 1b) to 400 hPa (Fig. 2) suppresses precursor diffusion into the trenches and therefore leads to a flat coalesced surface. Growth on such a coalesced Al0.45Ga0.55N layer allows for an increase in layer thickness and hence offers the possibility to fabricate substrates for UV-B LEDs.
S. Hagedorn, E. Richter, U. Zeimer, D. Prasai, W. John, M. Weyers "HVPE of AlxGa1-xN layers on planar and trench patterned sapphire" J. Cryst. Growth, vol. 353, no. 1, pp. 129-133 (2012).